docs: implementation plan for mapcodelib speed optimization

Step-by-step plan for Tasks 1-10 (baseline → A1-A5 → B3-B5 → results),
each with explicit file/line targets, code blocks, expected outputs,
and per-step commits.

Also fixes a small inaccuracy in the spec: RECORD_CODEX stores the
computed coDex value (10*(c/5)+(c%5+1)), not the raw flags & 31.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rijnb